Method of coating



Oct-

f \SPELLER I I METHOD OF COATING" Filed June 17, 1955 hwniar:

FEHNK N. QPE

Patented Oct. 11, 1938 PATENT OFFICE METHOD OF COATING Frank N. 'Speller, Pittsburgh, Pa., assignor to National Tube Company, a corporation of New Jersey Application June 17, 1935, Serial No. 27,104

1 Claim.

This invention relates to a method of coating. and more particularly to a method of" securing a continuous protective coating on ferrous metals.

It is a well known fact that practically all metal coatings on ferrous metals contain numerous,.but minute, pin holes.

. If the coating metal is cathodic to ferrous metal in contact with water there is a diiference of potential which tends to cause the solution of the ferrous metal wherever it is exposed. This condition is aggravated if the area of the exposed ferrous metal is very small in comparison with the area of the coating metal Coatings of lead and copper are exemplary of this reaction. In the case of zinc, by way of contrast, it is anodic to ferrous metals and therefore exerts a protective influence.

Under such a cathodic reaction, the ferrous metal becomes rapidly, and deeply, pitted wherever exposed,.and considerable damage often results.

I It is among the objects of the present invention to provide a novel method of securing 9. protec-'- tive coating on ferrous metals by clogging the numerous, but minute, fpin holes, thus stopping the corrosive reaction. I

This and other objects will be apparent after referring to the drawing, in which:

Figure l is a perspective ofa lead or copper coated pipe or tube which is pitted from exposure to a corrosive environment.

Figure 2 is a perspective of the pipe ortube of Figure 1 as treated in accordance with the method of the invention.

Figure 3 is a perspective of the pipe or tube of Figure 2 as further treated in accordance with a modification of the method of the invention.

Referring more particularly to the drawing, the numeral 2 designates a pipeor tubewhich is coated with lead, as at 3. by way of example. As previously mentioned, this coating contains numerous pin" holes 4.

According to the teachings of the invention, the pipe 2 with its lead coating 3 is placed in a suitable tank which contains a solution which is capable of forming an insoluble corrosion product with lead, and at the same time somehydroxide' of iron. The hydroxide of iron and the lead compounds formed tend to seal the "pin" holes 4 in the lead coating 3. This seal'is indicated at 6. As a specific example of treatment, this solution may be a dilute one (such as 5 percent) of sodium bicarbonate heated to a temperature of approximately degrees Fahrenheit.

The lead goes into solution in the water to ah, slight extent and the carbonate may be formed either by the presence of free carbon dioxide resulting from partial dissociation of sodium bicarbonate or by direct reaction of sodium carbonate or bicarbonate. In pipes used for steam return lines there is always present a certain amount of carbon diox-' ide and usually a small amount of oxygen. If a pipe intended for such a use is immersed for a suitable interval of time, such as five hours, in the dilute solution of sodium bicarbonate, the lead of the coating 3 is corroded, which results in the formation of lead carbonate (which is a very insoluble compound) and some little ferric hydroxide. The lead carbonate and ferric hydroxide serve admirably to seal the pin holes 4 in the coating against a corrosive environment.

Other compounds, such as dilute sulphuric acid, may be used to form insoluble lead salts, with equal facility; as another specific example, a dilute solution acid heated to a temperature of approximately I 130 degrees Fahrenheit.

Referring to Figure 3 of the drawing, a modifled form of the invention contemplates augmenting the coating 6 with a final surface coating 1 of some organic material, such as one of the v synthetic resin paints that is highly resistant to water.- This final coating 1 serves to further protect the pipe 2 and lead coating 3 from local electrolytic action,

suitable manner.

It is to be understood that a pipe which has been provided with a lead or other suitable coating and exposed to a corrosive environment can (such as one percent) of sulphuric and may be obtained in any be treated according to the method of the invention with the same facility as a new one. In the case of an exposed pipepthe pits I in the lead While I have shown and described'several' specific embodiments of my invention, it will be eating 3 may extend into the pipe itself, but will be eflectively sealed in the manner previously described. Y

be made without departing fromthe scope of my invention, as defined by the followingclaim.

I claim:

The method of treating a ferrous metal article which includes coating the same with a. metal which ls cathodic thereto, said coating being such that minute pin-holes are formed therein, and

then exposing said coated article toan approximate 5 percent solution of sodium bicarbonate at a temperature of approximately 130 degrees Fahrenheit, said solution being capable of producing free carbon dioxide and corroding said coating sufiiciently to clog the pin-holes therein. 

